U.S. patent number 6,081,641 [Application Number 08/962,862] was granted by the patent office on 2000-06-27 for thermal compensated fused-fiber dense wavelength division multiplexer.
This patent grant is currently assigned to Applied Fiber Optics, Inc.. Invention is credited to Peter Z. Chen.
United States Patent |
6,081,641 |
Chen |
June 27, 2000 |
Thermal compensated fused-fiber dense wavelength division
multiplexer
Abstract
A structure and method are provided to compensate for
temperature changes on wavelength shifting in DWDM by reducing
tension in a fused-fiber DWDM when temperature increases and
increasing tension when temperature decreases, thereby stabilizing
wavelength shifts throughout the range of operating temperatures.
The DWDM, which is enclosed by a protective package in some
embodiments, is connected to substrates of different thermal
expansion coefficients, such that the fused-fiber portion of the
DWDM exhibits negative thermal expansion, i.e. contracts when
temperature increases and expands when temperature decreases. As a
result, temperature-induced wavelength shifts are minimized due to
a passive thermal compensation, which can be easily adjusted. In
other embodiments, twisting the fused-fiber portion of the DWDM to
obtain optimum phase matching minimizes the polarization-dependent
losses in the incoming light.
Inventors: |
Chen; Peter Z. (Fremont,
CA) |
Assignee: |
Applied Fiber Optics, Inc.
(Fremont, CA)
|
Family
ID: |
25506435 |
Appl.
No.: |
08/962,862 |
Filed: |
November 3, 1997 |
Current U.S.
Class: |
385/43;
359/489.04 |
Current CPC
Class: |
G02B
6/29332 (20130101); G02B 6/29395 (20130101); G02B
6/29398 (20130101); G02B 6/2835 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); G02B 6/28 (20060101); G02B
006/26 () |
Field of
Search: |
;385/43,37,48,51,31,11,34,15,24,39 ;372/703,6,19,94,27
;359/494,123,110,124,127,161,121,181,189,191,156
;250/225,227.17 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 525 743 |
|
Feb 1993 |
|
EP |
|
97/26572 |
|
Jul 1997 |
|
WO |
|
Primary Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Skjerven, Morrill, MacPherson,
Franklin & Friel LLP Chen; Tom
Claims
I claim:
1. A fused-fiber dense wavelength division multiplexer (DWDM)
device, comprising:
a first substrate having a first thermal expansion coefficient;
a fused-fiber DWDM connected to said first substrate, wherein the
fused-fiber portion of said DWDM is under tension; and
a second substrate connected to said first substrate and having a
second thermal expansion coefficient, wherein said tension is
reduced when temperature rises and is increased when temperature
falls to provide passive thermal compensation.
2. The device of claim 1, wherein said DWDM is further connected to
said second substrate.
3. The device of claim 1, further comprising a third substrate
having a third thermal expansion coefficient and connected to said
DWDM and to said second substrate.
4. The device of claim 3, wherein said first and third thermal
expansion coefficients are equal.
5. The device of claim 3, wherein the fused-fiber portion of said
DWDM is connected between said first and third substrates.
6. The device of claim 3, wherein said tension is applied before
said DWDM is connected to said first and third substrates.
7. The device of claim 1, further comprising a package enclosing
said first substrate and said DWDM.
8. The device of claim 3, wherein said first thermal expansion
coefficient is higher than said second thermal expansion
coefficient.
9. The device of claim 1, wherein said DWDM has a channel spacing
of less than 7 nm at wavelengths around 1550 nm.
10. The device of claim 1, wherein said DWDM has a channel spacing
of less than 7 nm at wavelengths between 1200 and 1400 nm.
11. The device of claim 1, wherein said DWDM has a channel spacing
of less than 7 nm at wavelengths between 900 and 1200 nm.
12. The device of claim 1, wherein the fused-fiber portion of said
DWDM is formed by twisting said fused-fiber portion to obtain an
optimum phase matching condition.
13. The device of claim 12, wherein the fused-fiber portion of said
DWDM is twisted 0 to 2 turns.
14. The device of claim 1, wherein said tension is applied before
said DWDM is connected to said first substrate.
15. The device of claim 14, wherein the fused-fiber portion of said
DWDM is enclosed by said first substrate.
16. The device of claim 14, wherein said first and second
substrates are approximately the same length.
17. The device of claim 1, wherein the amount of thermal
compensation depends on the length of said first and second
substrates over which said first and second substrates are
connected.
18. The device of claim 14, wherein said first thermal expansion
coefficient is lower than said second thermal expansion
coefficient.
19. The device of claim 14, further comprising a third substrate
having a third thermal expansion coefficient higher than said
second thermal expansion coefficient and connected to said first
and second substrates.
20. The device of claim 19, wherein said third substrate expands to
force the middle portion of said first substrate to bend when
temperature increases.
21. The device of claim 19, wherein said third substrate expands to
force the end portions of said first substrate to bend when
temperature increases.
22. The device of claim 1, wherein the fused-fiber portion of said
DWDM is enclosed by said first substrate.
23. The device of claim 22, wherein said tension is applied after
said DWDM is connected to said first substrate by bending said
first substrate.
24. The device of claim 23, wherein said first substrate is
connected to the ends of a surface of said second substrate.
25. A fused-fiber dense wavelength division multiplexer (DWDM)
device, comprising:
a first substrate having a first thermal expansion coefficient;
a fused-fiber DWDM connected to said first substrate, wherein the
fused-fiber portion of said DWDM is under tension and enclosed by
said first substrate; and
a second substrate surrounding said first substrate and having a
second thermal expansion coefficient, wherein said tension is
reduced when temperature rises and is increased when temperature
falls to provide passive thermal compensation.
26. The device of claim 25, wherein said second substrate is bent
more than said first substrate.
27. A method of providing thermal compensation to a fused-fiber
dense wavelength division multiplexer (DWDM), comprising the steps
of:
applying tension to the fused-fiber portion of said DWDM;
connecting said DWDM to a first substrate having a first thermal
expansion coefficient; and
connecting a second substrate having a second thermal expansion
coefficient, wherein said tension is reduced when temperature rises
and is increased when temperature falls to provide passive thermal
compensation.
28. The method of claim 27, further comprising the step of
connecting a third substrate having a third thermal expansion
coefficient to said DWDM and to said second substrate.
29. The method of claim 27, further comprising the step of
connecting said DWDM to said second substrate.
30. The method of claim 27, wherein said first thermal expansion
coefficient is higher than said second thermal expansion
coefficient.
31. The method of claim 27, wherein the amount of thermal
compensation depends on the length of said first and second
substrates where said first and second substrates are
connected.
32. The method of claim 27, further comprising the step of twisting
the fused-fiber portion of said DWDM to obtain a optimum phase
matching condition.
33. The method of claim 27, further comprising the step of
enclosing the fused-fiber portion of said DWDM within said first
substrate.
34. The method of claim 33, further comprising the step of
connecting a third substrate having a third thermal expansion
coefficient to said second substrate and to a middle portion of
said first substrate to bend the middle portion when temperature
increases.
35. The method of claim 33, further comprising the step of
connecting a third substrate having a third thermal expansion
coefficient to said second substrate and to end portions of said
first substrate to bend the end portions when temperature
increases.
36. A method of providing thermal compensation to a fused-fiber
dense wavelength division multiplexer (DWDM), comprising the steps
of:
connecting said DWDM to a first substrate having a first thermal
expansion coefficient and enclosing the fused-fiber portion of said
DWDM within said first substrate;
applying tension to the fused portion of said DWDM by bending said
first substrate; and
connecting a second substrate having a second thermal expansion
coefficient to said first substrate, wherein said tension is
reduced when temperature rises and is increased when temperature
falls to provide passive thermal compensation.
37. The method of claim 36, wherein said first substrate is
connected to the ends of a surface of said second substrate.
38. The method of claim 36, wherein said first thermal expansion
coefficient is lower than said second expansion coefficient.
39. A method of providing thermal compensation to a fused-fiber
dense wavelength division multiplexer (DWDM), comprising the steps
of:
connecting said DWDM to a first substrate having a first thermal
expansion coefficient and enclosing the fused-fiber portion of said
DWDM within said first substrate;
applying tension to the fused portion of said DWDM by bending said
first substrate; and
inserting said first substrate into a second substrate having a
second thermal expansion coefficient, wherein said tension is
reduced when temperature rises and is increased when temperature
falls to provide passive thermal compensation.
40. The method of claim 39, wherein said second substrate is bent
more than said first substrate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fused-fiber wavelength division
multiplexers (WDM) and, in particular, to dense WDM (DWDM).
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to
increase information transmission capacity. However, both physical
and economic constraints can limit the feasibility of increasing
transmission capacity. For example, installing additional fiber
optic cable to support additional wavelengths can be cost
prohibitive, and electronic system components may impose physical
limitations on the amount of information that can be transmitted.
The use of wavelength division multiplexers (WDMS) provides a
simple and economical way to increase the transmission capacity of
fiber optic communication systems by allowing multiple wavelengths
to be transmitted and received over a single optical fiber.
WDMs can be manufactured using, for example, biconical tapered
fusion (BTF) technology. Typically, two optical fibers are fused
together along an interior portion to form a fused-fiber coupler,
such that light of two wavelengths (i.e., 1310 nm and 1550 nm)
entering the inputs of the first and second fibers, respectively,
are multiplexed onto a single fiber, transmitted, and then
demultiplexed onto the two outputs of the first and second fibers.
Light at 1550 nm is particularly desirable because minimal
absorption is exhibited by optical fibers around this wavelength.
Commercially available fused-fiber WDMs typically also couple and
decouple light at 1550 nm and 980 nm and at 1550 nm and 1480
nm.
The principles of WDM can be extended to further increase data
transmission capability by coupling additional discrete wavelengths
or channels onto a single fiber using a more recent technology
known as dense WDM (DWDM). Fused-fiber DWDMs may couple, for
example, 8, 16, or even 32 discrete communication channels onto a
single optic fiber. However, because the usable bandwidth of the
light is limited, increasing the number of wavelengths necessarily
results in smaller channel separation between the discrete
wavelengths. In general, smaller channel spacing can be achieved by
increasing the length of the fused portion of a fused-fiber DWDM.
However, decreasing channel spacing presents different types of
problems, such as increased sensitivity to temperature
fluctuations.
As temperature increases in the fused-fiber WDM, the refractive
index of the fused-fiber portion increases due to the refractive
index dependence on temperature of fused silica, which is
approximately 6.times.10.sup.-6 /.degree. C. This causes a longer
optical path inside the coupling region of the WDM, and therefore,
shifts the peak transmission wavelengths toward shorter
wavelengths, thereby decreasing the channel spacing or wavelength
separation capability of the WDM. These temperature-induced shifts
normally do not adversely affect conventional WDMs, which typically
have channel spacings of 50 nm or more. However, with DWDMs,
typically having channel spacings of 1 nm or less, such wavelength
shifts can pose significant problems with transmission
performance.
One method to reduce temperature-induced wavelength shifts is to
use a thermal-isolated device containing an electric cooler/heater
with a temperature feedback element to actively maintain a constant
temperature of the fused-fiber DWDM structure. However, such a
device adds size, as well as cost and complexity, to the
multiplexer, and thus the fiber optic communication system.
Furthermore, over a wide temperature range, such as -40.degree. C.
to approximately 80.degree. C., the device's ability to maintain a
constant temperature decreases.
Accordingly, a structure and method are desired for controlling
temperature-induced effects in fused-fiber DWDMs which overcomes
the disadvantages discussed above with conventional devices.
SUMMARY OF THE INVENTION
The present invention provides a structure and method to compensate
for temperature-induced wavelength shifts in fused-fiber dense
wavelength division multiplexers (DWDMS) by relieving tension to
the DWDM when temperature increases and increasing tension when
temperature decreases. Polarization-dependent losses can also be
minimized by twisting the fused-fiber portion of the DWDM to attain
optimum phase matching between two incoming light sources.
A conventional fused-fiber DWDM is secured to a first substrate,
where the fused-fiber coupling portion of the DWDM is under
tension. The amount of tension is adjusted to set the desired pass
band wavelengths of the incoming light. In some embodiments, the
tension is applied prior to securing the DWDM to the first
substrate; while in other embodiments, tension is applied
afterwards by bending the first substrate together with the
fused-fiber portion of the DWDM. The first substrate is then
connected to a second substrate at a plurality of mounting points
in such a manner that the distance between mounting points
decreases when temperature increases, which releases tension in the
DWDM. The second substrate can be
a single substrate or a composite of two or more substrates, where
the second substrate has a thermal expansion coefficient higher
than that of the first substrate.
Due to the different rates of thermal expansion, the fused-fiber
portion of the DWDM expands and contracts with changes in
temperature, which shifts the wavelength peak to shorter and longer
wavelengths, respectively. Thus, when temperature increases, this
passive thermal compensation (PTC) relieves tension in the DWDM to
shift the wavelength towards higher wavelengths, which counteracts
temperature-induced wavelength shifts toward shorter
wavelengths.
Tension can also released as temperature increases by bending the
middle portion of the DWDM in some embodiments, while bending the
end portions of the DWDM in other embodiments. The amount of
compensation, which depends on the amount of tension released, can
be easily adjusted by changing the connection points of the
substrates, the length at which the two substrates are connected,
the relative thermal expansion coefficients of the substrates, or
the thickness and length of the substrates, depending on the
particular embodiment.
In other embodiments, where tension is applied to the DWDM by
bending the DWDM and first substrate, the first substrate is
inserted into a bent second substrate. The second substrate is bent
more than the first substrate so that the second substrate
maintains the desired bend in the first substrate without physical
connections such as by epoxy. The second substrate also has a
thermal expansion coefficient higher than the first substrate,
which results in the first substrate unbending and releasing
tension as temperature increases to provide the desired passive
thermal compensation.
Accordingly, temperature-induced wavelength shifting in fused-fiber
DWDMs can be quickly and easily compensated for by increasing or
decreasing the amount of tension in the fused-fiber coupling
portion of the DWDM using passive thermal compensation.
This invention will be more fully understood in light of the
following detailed description taken together with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a conventional fused-fiber WDM;
FIG. 2 is a diagram of a WDM secured within a sleeve;
FIG. 3 is a diagram of the structure of FIG. 2 enclosed within a
protective package;
FIG. 4A is a graph of a typical spectrum of the output light from a
WDM;
FIG. 4B is a graph of a typical spectrum of the output light from a
WDM with optimum phase matching (OPM);
FIG. 5 is a diagram of a DWDM according to one embodiment of the
present invention utilizing OPM;
FIG. 6A is a diagram of a DWDM according to one embodiment of the
present invention utilizing passive thermal compensation;
FIG. 6B is a diagram of the DWDM of FIG. 6A enclosed in a
protective package;
FIGS. 7A are 7B are graphs showing wavelength dependencies on
temperature and tension, respectively; and
FIGS. 8A, 8B, 9A, 9B and 10-13 are diagrams of DWDM devices
according to other embodiments of the present invention.
Use of the same reference numbers in different figures indicates
similar or like elements.
DETAILED DESCRIPTION
The present invention discloses a structure and method to
compensate for shifts in the peak wavelength, or equivalently in
the refractive index, caused by temperature variations in
fused-fiber dense wavelength division multiplexers (DWDMs) using
passive thermal compensation (PTC) techniques.
A typical fused-fiber WDM 10, also known as a biconical tapered
fusion (BTF) coupler, is shown in FIG. 1. Fused-fiber WDM 10 can be
formed by heating and axially stretching two optic fibers 11 and 12
and fusing the two fibers together along a predetermined length to
form a coupling region 13. Broadband light at two wavelengths,
entering WDM 10 at input ports 14 and 15, couple onto and travel
along fused coupling region 13, and then decouple and exit WDM 10
at output ports 16 and 17, respectively.
To provide mechanical protection, as well as thermal stabilization,
WDM 10 is typically placed or secured within a quartz sleeve
substrate 20, as shown in FIG. 2. Thermal stabilization is achieved
because quartz has a thermal expansion coefficient similar to that
of the silica fiber. WDM 10 and quartz sleeve 20 can then be placed
inside a sealed package 30, typically metal, to increase mechanical
and humidity protection for the fused-fiber multiplexer, as shown
in FIG. 3.
FIG. 4A shows a typical spectrum of the output light from a WDM
after traveling through the fused-fiber coupling region (for
example, from output ports 16 and 17 of FIG. 1, after traveling
through coupling region 13 of WDM 10). Light from one output is
shown by solid line 41, and the simultaneous light from the other
output is shown by dashed line 42. The transmission curve consists
of alternating peaks and nulls which result from the transverse
mode interference in the coupling region. The channel spacing,
defined as the wavelength spacing between the peak 43 of a
wavelength from one output port and the peak 44 of the nearest
wavelength from a second output port, is the wavelength separation
(or combination) capability of the WDM. The channel spacing of most
commonly available commercial fused-fiber WDMs are typically 50 nm
or more, i.e., 70 nm for a 1480 nm/1550 nm pair, 240 nm for a 1310
nm/1550 nm pair, and 570 nm for a 980 nm/1550 nm pair.
However, the wavelength peaks and pass bands shift as the
temperature of the WDM changes due to the temperature dependence of
the refractive index of fused silica (.about.6.times.10.sup.-6
/.degree. C.). For example, experiments have shown that the
wavelength peaks in fused-fiber WDMs change with temperature at a
rate of approximately 0.012 nm/.degree. C. for light at wavelengths
around 1550 nm. Thus, for a 100.degree. C. change in temperature,
the shift will be about 1.2 nm. Such a temperature-induced
wavelength shift is normally insignificant for conventional BTF
WDMs due to the large separation between channels or wavelength
peaks, i.e., 50 nm or more.
However, in fused-fiber DWDMs, the signal light (for example,
within the operation bandwidth of Er-doped fiber amplifiers) might
be divided into multiple wavelengths for multi-channel
transmission, i.e., 8 or more different channels. Co-owned U.S.
patent app. Ser. No. 08/799,582, entitled "Apparatus and Method of
Making a Fused Dense Wavelength Division Multiplexer", filed on
Feb. 12, 1997, now U.S. Pat. No. 5,809,190, which is incorporated
herein by reference in its entirety, discloses a method of
increasing the number of channels in DWDMs. Because the usable
wavelength bandwidth of the amplifiers is limited (e.g., light at
1550 nm has a usable bandwidth of approximately 20 to 30 nm), the
channel spacing decreases as the number of channels increases. For
example, with a 32-channel DWDM, the channel spacing is only
.about.0.8 nm, and thus a potential temperature-induced 1.2 nm
wavelength shift can pose significant problems for a fused-fiber
DWDM.
The transmission curve of FIG. 4A also exhibits a slower modulating
envelope resulting from polarization mode interference. Because the
fused-fiber in the coupling region is birefringent, i.e., the phase
velocity of the light propagating through the coupling region is
dependent on the direction of polarization of the light, losses due
to polarization are maximum in the lowest parts 45 of the
modulation envelope and minimum in the highest parts 43 and 44 of
the modulation envelope.
In one embodiment of the present invention, shown in FIG. 5, a DWDM
50 utilizes optimum phase matching (OPM) to minimize the effects of
birefringence. For OPM, a 2.pi. phase difference exists between two
orthogonally polarized lights. An optimum phase match is then
obtained by twisting the pair of optic fibers 51 and 52 a few turns
along coupling region 53 before or after the fusion process.
Typically, a range of 0 to 2 turns is sufficient to achieve an OPM,
depending on the length of the fused coupling region and the
wavelengths of the incoming light. By utilizing optimum phase
match, a wider modulation envelope and more pass bands of
approximately equal amplitude (typically 5 or more) are possible,
as shown in FIG. 4B.
FIG. 6A shows one embodiment according to the present invention for
compensating for temperature-induced wavelength shifts in a DWDM. A
composite structure 60 is formed by first mounting a fused-fiber
DWDM 61 on a first substrate 62 and a second substrate 63, both
made of materials with high thermal expansion coefficients, such as
steel or aluminum. Substrates 62 and 63 are mounted onto a third
substrate 64, made of a material with a lower thermal expansion
coefficient than substrates 62 and 63, such as quartz or invar.
Tension is applied to the fused coupling region 65 of DWDM 61,
which is then placed between the inner ends of substrates 62 and 63
and secured thereon at mounting points 66 by epoxy or other
suitable means, such as soldering, laser welding, or clamps. The
amount of tension applied is determined based on the refractive
index change of the fibers and the thermal expansion coefficients
of substrates 62-64. Epoxy or other suitable means is also used to
secure substrates 62 and 63 with substrate 64 at mounting points
67. Composite structure 60 can then be placed in a protective
housing 68, as shown in FIG. 6B.
Composite structure 60 minimizes temperature-induced wavelength
drifts by using passive thermal compensation (PTC). Due to the
refractive index increase of the fused-fiber, the pass band or peak
wavelength of the fused-fiber DWDM is shifted towards shorter
wavelengths as temperature rises, as shown in FIG. 7A.
Concurrently, substrates 62-64 of FIG. 6A extend their lengths in
accordance with their respective thermal expansion coefficients. As
a result, substrates 62 and 63 expand more than substrate 64 due to
the higher thermal expansion coefficients of substrates 62 and 63
(as shown by the size of the arrows in FIG. 6A). Because substrates
62 and 63 are secured to substrate 64 at mounting points 67, the
distance between the two mounting points 66 decreases.
Equivalently, the length of fused coupling region 65 between
mounting points 66 is shortened as temperature rises so that, in
effect, the substrate made of substrates 62-64 acts as one with a
negative thermal expansion coefficient.
As the distance between mounting points 66 decreases, the tension
on fused portion 65 is reduced, which shifts the pass band or peak
wavelength towards longer wavelengths, as shown in FIG. 7B, because
the refractive index of silica fibers also has a positive tension
dependence. Therefore, decreases in wavelength due to a rise in
temperature are compensated for by simultaneously increasing the
wavelength as a result of a reduction in tension. Similarly, when
temperature falls, substrates 62 and 63 contract more than
substrate 64, thereby increasing the tension on fused portion 65
and compensating for wavelength shifts due to a temperature
decrease. As a result, the problem of temperature-induced
wavelength shifts is minimized with small, inexpensive, and simple
composite DWDMs utilizing passive thermal compensation.
By adjusting the amount of compensation provided by the DWDM, the
wavelength along the fused portion of the DWDM can be kept
relatively constant throughout the range of operating temperatures.
In the present embodiment, the amount of compensation can be easily
changed by moving mounting points 67 at which substrates 62 and 63
are joined to substrate 64, with the amount of compensation
increasing as mounting points 67 are placed closer to the ends of
substrate 62 (and/or substrate 63) and substrate 64. The amount of
compensation can also be changed by moving mounting points 66 at
which DWDM 61 connects to substrates 62 and 63, with compensation
increasing as mounting points 66 are moved closer together. It
should also be noted that the pass band or wavelength peak can be
adjusted by changing the tension to fused portion 65 before
securing DWDM 61 to substrates 62 and 63, i.e., increasing tension
moves the pass band to shorter wavelengths and decreasing tension
moves the pass band to longer wavelengths. The amount of tension
cannot be increased such that the multiplexer breaks at the lower
limit of operating temperatures due to the contraction of
substrates 62 and 63; while the amount of tension should not be
decreased to the point that tension reaches zero prior to the upper
limit of the operating temperatures from the expansion of
substrates 62 and 63.
Thus, the present invention provides a method and structure for
fused-fiber DWDMs which minimizes the problematic effects of
temperature-induced wavelength shifts in DWDM. Consequently, using
DWDMs of the present invention, channel spacings of less than 7 nm,
i.e., at 0.8 nm, 1.6 nm, 3.2 nm, or 6.4 nm, are possible at
wavelengths around 1550 nm. Furthermore, channel spacings of less
than 7 nm are also possible at wavelengths in the ranges of
900-1200 nm and 1200-1400 nm. In addition, the embodiment shown in
FIG. 6A allows both the tension and amount of compensation to be
easily changed or adjusted by simply changing the location of
mounting points between substrates 62 and 63 and substrate 64
and/or between substrates 62 and 63 and DWDM 61.
It will also be appreciated by those skilled in the art that
substrates 62 or 63 can have the same or different thermal
expansion coefficients as long as the thermal expansion
coefficients of both substrates 62 and 63 together are relatively
higher than the thermal expansion coefficient of substrate 64.
Furthermore, one or both of substrates 62 and 63 can be replaced by
epoxy or other securing material having a higher thermal expansion
coefficient than substrate 64 to connect one or both end portions
of fused coupling region 65 directly to substrate 64, as shown in
FIGS. 8A and 8B, respectively. In all cases, the composite
structure of the substrates must provide enough negative thermal
expansion between mounting points 81 to adequately compensate for
wavelength shifts in the range of operating temperatures.
In addition to movement in the horizontal direction to change
length, substrates 62-64 in FIG. 6A also move in the vertical
direction to change thickness in response to temperature changes.
Thus, if substrates 62 and 63 have a different thickness or are
made of different materials, the relative vertical separation
between substrates 62 and 63 will also change. Consequently, the
vertical separation between mounting points 66 changes with
temperature, thereby altering the tension of fused coupler region
65. Therefore, the degree of wavelength compensation can be
adjusted by changing the relative thicknesses and/or thermal
expansion coefficients of substrates 62 and 63 to change the angle
between fused coupling region 65 and substrates 62 and 63 along the
direction of vertical movement.
In other embodiments of the present invention, passive thermal
compensation can be achieved with pre-existing fused-fiber DWDMs
and packages, thereby potentially reducing costs, time, and
complexity. In one set of embodiments, shown in FIGS. 9-11, tension
is first applied to the coupling region of fused-fiber DWDM 61, and
then DWDM 61 is secured to a protective substrate 90, such as the
sleeve shown in FIG. 2. Substrate 90 is connected to a structure
which bends substrate 90 as temperature increases, thereby
releasing tension in DWDM 61 for PTC. In another set of
embodiments, shown in FIGS. 12-13, tension is applied to the
coupling region after DWDM 61 is secured to substrate 90. In these
embodiments, substrate 90 is first bent to provide tension within
the coupling region of DWDM 61, and then the bent substrate is
attached to a structure which unbends substrate 90 as temperature
increases to provide the desired PTC.
In FIG. 9A, substrate 90 is attached lengthwise to a second
substrate 91 at mounting point 92 by an epoxy or any suitable
connection means. Substrate 91 can be attached to either the upper
or lower face of substrate 90 having a lower thermal expansion
coefficient than substrate 91. As temperature rises, substrate 91
expands more than substrate 90, causing the ends of both substrates
90 and 91 to bend, as shown in FIG. 9B, which releases some of the
tension in the coupling region of DWDM 61 to provide the desired
PTC. The amount of compensation can be adjusted by varying the
length over which substrates 90 and 91 are connected. Increasing
the length of substrate 90 that is attached to substrate 91
increases the amount of temperature compensation available.
In FIG. 10, substrate 90, preferably square or rectangular shaped,
is attached at its ends to the ends of a U-shaped substrate 101 and
at its middle to one end of a substrate 102, which has its other
end attached to a middle portion of substrate 101. Substrate 102
has a higher thermal expansion coefficient than substrate 101.
Therefore, as temperature increases, substrate 102 expands more
than substrate 101, which bends substrate 90 and releases tension
from DWDM 61 to provide PTC. In FIG. 11, substrate 90, again
preferably square or rectangular shaped, is attached at its middle
to a substrate 111 at point 113 and at its ends to a substrate 112
having a higher thermal expansion coefficient than substrate 111.
Substrate 112 is positioned against substrate 111 such that when
temperature increases, substrate 112 expands to bend substrate 90
and lessen tension in DWDM 61.
FIG. 12 shows an embodiment of the present invention, where no
tension is applied to DWDM 61 prior to placement within protective
substrate 90. Substrate 90 is bent to provide tension within the
fused coupling region of DWDM 61 and then inserted in a curved
substrate 121, such as the steel tube shown in FIG. 12. The
curvature of substrate 121 must be more than that of substrate 90
to keep substrate 90 bent once substrate 90 is inserted into
substrate 121. Thus, tight contact is maintained at outer points
122 and a middle point 123. Substrate 121 also has a higher thermal
expansion coefficient than substrate 90 so that when temperature
increases, substrate 121 expands more than substrate 90, thereby
releasing the bending of substrate 90, which relieves the tension
of DWDM 61 to provide PTC.
In FIG. 13, an embodiment is shown utilizing a concept similar to
that described with respect to the embodiment of FIG. 12. Substrate
90 is bent to provide tension to the fused portion of DWDM 61 and
then secured at the ends of the upper surface of a substrate 131 at
mounting points 132 by epoxy or other suitable means to maintain
the tension. Substrate 131 has a higher thermal expansion
coefficient than substrate 90. Therefore, as temperature rises,
substrate 131 expands more rapidly than substrate 90, which unbends
substrate 90 and releases tension in DWDM 61, as indicated by the
relative sizes of the arrows in FIG. 13. Similarly, as temperature
decreases, substrate 131 contracts more rapidly to create
additional tension in the fused portion of DWDM 61 by bending
substrate 90 and DWDM 61. This bending and unbending compensates
for the temperature effects on wavelength shifting. The amount of
compensation can be varied by changing the relative lengths of
substrates 90 and 131, where lengthening substrate 131 relative to
substrate 90 increases the amount of temperature compensation.
The above-described embodiments of the present invention are merely
meant to be illustrative and not limiting. It will thus be obvious
to those skilled in the art that various changes and modifications
may be made without departing from this invention in its broader
aspects. Therefore, the appended claims encompass all such changes
and modifications as fall within the true spirit and scope of this
invention.
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